Am 02.06.2020 um 14:18 hat Sergio Lopez geschrieben: > On Tue, Jun 02, 2020 at 01:23:14PM +0200, Kevin Wolf wrote: > > Am 02.06.2020 um 10:11 hat Sergio Lopez geschrieben: > > > Disable request queuing while switching contexts on > > > virtio_blk_data_plane_[start|stop](), preventing requests from getting > > > queued on the wrong context. > > > > > > Placing requests on the wrong context may lead to them being wrongly > > > accessed in parallel from different threads, potentially leading to > > > multiple issues. > > > > > > For example, stopping and resuming a VM multiple times while the guest > > > is generating I/O on a virtio_blk device can trigger a crash with a > > > stack tracing looking like this one: > > > > > > <------> > > > Thread 2 (Thread 0x7ff736765700 (LWP 1062503)): > > > #0 0x00005567a13b99d6 in iov_memset > > > (iov=0x6563617073206f4e, iov_cnt=1717922848, offset=516096, fillc=0, > > > bytes=7018105756081554803) > > > at util/iov.c:69 > > > #1 0x00005567a13bab73 in qemu_iovec_memset > > > (qiov=0x7ff73ec99748, offset=516096, fillc=0, > > > bytes=7018105756081554803) at util/iov.c:530 > > > #2 0x00005567a12f411c in qemu_laio_process_completion > > > (laiocb=0x7ff6512ee6c0) at block/linux-aio.c:86 > > > #3 0x00005567a12f42ff in qemu_laio_process_completions > > > (s=0x7ff7182e8420) at block/linux-aio.c:217 > > > #4 0x00005567a12f480d in ioq_submit (s=0x7ff7182e8420) at > > > block/linux-aio.c:323 > > > #5 0x00005567a12f43d9 in qemu_laio_process_completions_and_submit > > > (s=0x7ff7182e8420) > > > at block/linux-aio.c:236 > > > #6 0x00005567a12f44c2 in qemu_laio_poll_cb (opaque=0x7ff7182e8430) at > > > block/linux-aio.c:267 > > > #7 0x00005567a13aed83 in run_poll_handlers_once (ctx=0x5567a2b58c70, > > > timeout=0x7ff7367645f8) > > > at util/aio-posix.c:520 > > > #8 0x00005567a13aee9f in run_poll_handlers (ctx=0x5567a2b58c70, > > > max_ns=16000, timeout=0x7ff7367645f8) > > > at util/aio-posix.c:562 > > > #9 0x00005567a13aefde in try_poll_mode (ctx=0x5567a2b58c70, > > > timeout=0x7ff7367645f8) > > > at util/aio-posix.c:597 > > > #10 0x00005567a13af115 in aio_poll (ctx=0x5567a2b58c70, blocking=true) > > > at util/aio-posix.c:639 > > > #11 0x00005567a109acca in iothread_run (opaque=0x5567a2b29760) at > > > iothread.c:75 > > > #12 0x00005567a13b2790 in qemu_thread_start (args=0x5567a2b694c0) at > > > util/qemu-thread-posix.c:519 > > > #13 0x00007ff73eedf2de in start_thread () at /lib64/libpthread.so.0 > > > #14 0x00007ff73ec10e83 in clone () at /lib64/libc.so.6 > > > > > > Thread 1 (Thread 0x7ff743986f00 (LWP 1062500)): > > > #0 0x00005567a13b99d6 in iov_memset > > > (iov=0x6563617073206f4e, iov_cnt=1717922848, offset=516096, fillc=0, > > > bytes=7018105756081554803) > > > at util/iov.c:69 > > > #1 0x00005567a13bab73 in qemu_iovec_memset > > > (qiov=0x7ff73ec99748, offset=516096, fillc=0, > > > bytes=7018105756081554803) at util/iov.c:530 > > > #2 0x00005567a12f411c in qemu_laio_process_completion > > > (laiocb=0x7ff6512ee6c0) at block/linux-aio.c:86 > > > #3 0x00005567a12f42ff in qemu_laio_process_completions > > > (s=0x7ff7182e8420) at block/linux-aio.c:217 > > > #4 0x00005567a12f480d in ioq_submit (s=0x7ff7182e8420) at > > > block/linux-aio.c:323 > > > #5 0x00005567a12f4a2f in laio_do_submit (fd=19, laiocb=0x7ff5f4ff9ae0, > > > offset=472363008, type=2) > > > at block/linux-aio.c:375 > > > #6 0x00005567a12f4af2 in laio_co_submit > > > (bs=0x5567a2b8c460, s=0x7ff7182e8420, fd=19, offset=472363008, > > > qiov=0x7ff5f4ff9ca0, type=2) > > > at block/linux-aio.c:394 > > > #7 0x00005567a12f1803 in raw_co_prw > > > (bs=0x5567a2b8c460, offset=472363008, bytes=20480, > > > qiov=0x7ff5f4ff9ca0, type=2) > > > at block/file-posix.c:1892 > > > #8 0x00005567a12f1941 in raw_co_pwritev > > > (bs=0x5567a2b8c460, offset=472363008, bytes=20480, > > > qiov=0x7ff5f4ff9ca0, flags=0) > > > at block/file-posix.c:1925 > > > #9 0x00005567a12fe3e1 in bdrv_driver_pwritev > > > (bs=0x5567a2b8c460, offset=472363008, bytes=20480, > > > qiov=0x7ff5f4ff9ca0, qiov_offset=0, flags=0) > > > at block/io.c:1183 > > > #10 0x00005567a1300340 in bdrv_aligned_pwritev > > > (child=0x5567a2b5b070, req=0x7ff5f4ff9db0, offset=472363008, > > > bytes=20480, align=512, qiov=0x7ff72c0425b8, qiov_offset=0, flags=0) at > > > block/io.c:1980 > > > #11 0x00005567a1300b29 in bdrv_co_pwritev_part > > > (child=0x5567a2b5b070, offset=472363008, bytes=20480, > > > qiov=0x7ff72c0425b8, qiov_offset=0, flags=0) > > > at block/io.c:2137 > > > #12 0x00005567a12baba1 in qcow2_co_pwritev_task > > > (bs=0x5567a2b92740, file_cluster_offset=472317952, offset=487305216, > > > bytes=20480, qiov=0x7ff72c0425b8, qiov_offset=0, l2meta=0x0) at > > > block/qcow2.c:2444 > > > #13 0x00005567a12bacdb in qcow2_co_pwritev_task_entry > > > (task=0x5567a2b48540) at block/qcow2.c:2475 > > > #14 0x00005567a13167d8 in aio_task_co (opaque=0x5567a2b48540) at > > > block/aio_task.c:45 > > > #15 0x00005567a13cf00c in coroutine_trampoline (i0=738245600, i1=32759) > > > at util/coroutine-ucontext.c:115 > > > #16 0x00007ff73eb622e0 in __start_context () at /lib64/libc.so.6 > > > #17 0x00007ff6626f1350 in () > > > #18 0x0000000000000000 in () > > > <------> > > > > > > This is also known to cause crashes with this message (assertion > > > failed): > > > > > > aio_co_schedule: Co-routine was already scheduled in 'aio_co_schedule' > > > > > > RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1812765 > > > Signed-off-by: Sergio Lopez <s...@redhat.com> > > > --- > > > hw/block/dataplane/virtio-blk.c | 10 ++++++++++ > > > 1 file changed, 10 insertions(+) > > > > > > diff --git a/hw/block/dataplane/virtio-blk.c > > > b/hw/block/dataplane/virtio-blk.c > > > index 1b52e8159c..f1c7ba69c0 100644 > > > --- a/hw/block/dataplane/virtio-blk.c > > > +++ b/hw/block/dataplane/virtio-blk.c > > > @@ -214,12 +214,17 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev) > > > vblk->dataplane_started = true; > > > trace_virtio_blk_data_plane_start(s); > > > > > > + /* Prevent requests from getting queued on the old context */ > > > + blk_set_disable_request_queuing(s->conf->conf.blk, true); > > > + > > > r = blk_set_aio_context(s->conf->conf.blk, s->ctx, &local_err); > > > if (r < 0) { > > > error_report_err(local_err); > > > goto fail_guest_notifiers; > > > } > > > > > > + blk_set_disable_request_queuing(s->conf->conf.blk, false); > > > + > > > > Why are we even getting new requests that could possibly be queued? This > > is in virtio_blk_data_plane_start/stop(), so clearly the device should > > be idle and the virtio queues shouldn't be processed at all at the same > > time? > > Requests are processed and queued by virtio_blk_dma_restart_bh(), > which is a BH registered by virtio_blk_dma_restart_cb(), which in turn > is a callback for VM state change events registered with > qemu_add_vm_change_state_handler(virtio_blk_dma_restart_cb, s). > > This is the back trace of a request being queued this way: > > (gdb) bt > #0 0x000055cd35ce7490 in virtio_blk_submit_multireq (blk=0x55cd38cd24d0, > mrb=0x7ffd25b365c0) > at /root/src/qemu/hw/block/virtio-blk.c:448 > #1 0x000055cd35ce7d5f in virtio_blk_handle_request (req=0x7f5a4c022c80, > mrb=0x7ffd25b365c0) > at /root/src/qemu/hw/block/virtio-blk.c:686 > #2 0x000055cd35ce8602 in virtio_blk_dma_restart_bh (opaque=0x55cd38ccf4c0) > at /root/src/qemu/hw/block/virtio-blk.c:836 > #3 0x000055cd360dc4f5 in aio_bh_call (bh=0x55cd3880b300) at > /root/src/qemu/util/async.c:164 > #4 0x000055cd360dc4f5 in aio_bh_poll (ctx=ctx@entry=0x55cd3780bff0) at > /root/src/qemu/util/async.c:164 > #5 0x000055cd360dfd29 in aio_poll (ctx=ctx@entry=0x55cd3780bff0, > blocking=blocking@entry=true) > at /root/src/qemu/util/aio-posix.c:650 > #6 0x000055cd3603a645 in bdrv_do_drained_begin > (poll=true, ignore_bds_parents=false, parent=0x0, recursive=false, > bs=0x55cd3788d550) > at /root/src/qemu/block/io.c:429 > #7 0x000055cd3603a645 in bdrv_do_drained_begin > (bs=0x55cd3788d550, recursive=<optimized out>, parent=0x0, > ignore_bds_parents=<optimized out>, poll=<optimized out>) at > /root/src/qemu/block/io.c:395 > #8 0x000055cd35fde969 in bdrv_set_aio_context_ignore > (bs=bs@entry=0x55cd3788d550, > new_context=new_context@entry=0x55cd37810e40, > ignore=ignore@entry=0x7ffd25b36890) at /root/src/qemu/block.c:6309 > #9 0x000055cd35fdef33 in bdrv_child_try_set_aio_context > (bs=bs@entry=0x55cd3788d550, ctx=0x55cd37810e40, ignore_child=<optimized > out>, errp=<optimized out>) > at /root/src/qemu/block.c:6431 > #10 0x000055cd3602919b in blk_do_set_aio_context > (blk=0x55cd38cd24d0, new_context=0x55cd37810e40, > update_root_node=<optimized out>, errp=<optimized out>) at > /root/src/qemu/block/block-backend.c:2016 > #11 0x000055cd35cef54f in virtio_blk_data_plane_start (vdev=<optimized out>) > at /root/src/qemu/hw/block/dataplane/virtio-blk.c:220
Ugh. Thanks, I see now what's going on. We were relying on blk_set_aio_context() to complete the requests, and since BlockBackend queues the requests instead of completing them during drained_begin, we end up executing the requests after drained_end in the main loop context, but the block nodes have meanwhile moved to the iothread context. Of course, this was already a bad hack before, witnessed by the FIXME comment in virtio_blk_dma_restart_cb(). > I'm not familiar with the code, but I guess we need this BH to ensure > we process the virtqueue after resuming. The BH was introduced in commit 213189ab65d. The commit message has a very detailed problem description, but to summarise, if I understand correctly, the main purpose is that we don't end up stopping the VM due to an I/O error while we are only in the middle of sending VM state handlers events that the VM is about to be continued. > We could perhaps schedule the BH in a different way, or add a flag to > VirtIOBlock so virtio_blk_data_plane_[start|stop] would instruct > virtio_blk_dma_restart_bh() to avoid processing the virtqueue, > rescheduling itself instead. How does this would look to you? Yes, I think if dataplane is enabled, we should really run the restarted requests in the iothread. virtio_blk_dma_restart_cb() is called too early, before the dataplane is running, so I guess we need some way to tell the dataplane that it should run the request restarting logic of virtio_blk_dma_restart_bh() after it has started (no additional BH needed there). Your flag sounds like the obvious way to get this. In fact, would it hurt to do this just unconditionally without a flag? s->rq should be NULL when there is nothing to restart. And then we can skip the same logic in virtio_blk_dma_restart_cb() if we know that a dataplane is going to be started. Kevin
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